This invention relates generally to circuitry for use in current sensing applications where it is desired to produce an electrically isolated control signal output dependent on the magnitude of an input current, particularly in (but not limited to) applications where the maximum input current encountered may be one or more orders of magnitude greater than the minimum input current that must be sensed.
As an example of applications where such circuit requirements and input conditions exist, reference can be made to certain continuity monitoring circuits used in connection with high voltage circuit breakers. Where the speed of activation of the circuit breaker is important, it is common to provide for voltage forcing of the circuit breaker trip solenoid which causes a relatively high activation current (trip current) to pass through the solenoid. Such high activation current may be substantially higher than the minimum current necessary to cause activation.
Circuit breakers of this type are often used to protect very expensive equipment, hence it is important that they operate reliably when called upon to do so. However, they may not be called upon to operate for extended periods of time. In the interim, the electrical continuity of the trip solenoid may, by some mechanism, become damaged or otherwise deteriorate. A common method of detecting that such damage or deterioration may have occurred is to provide for the passage of a relatively low current (continuity current) through the trip solenoid and continuously sense the presence or absence of such current. The continuing presence of a continuity current is generally a reliable indication that the circuit breaker will activate properly if so required. Conversely, the absence of continuity current is generally a reliable indication that a fault may have developed in the trip solenoid. A function of a continuity monitoring circuit is to sense the presence or absence of continuity current and provide a warning or suitable output control signal when the input current drops below the desired level of continuity current.
The magnitude of the continuity current is an externally controlled predetermined minimum value which must necessarily be less than the drop-out current threshold of the trip solenoid. Owing to hysterisis effects, the drop-out current threshold may be considerably less than the previously mentioned minimum activation current.
Hence, the current sensing circuitry of the continuity monitoring circuit must reliably sense a relatively low continuity current (ie. 10 millamperes) and must tolerate relatively high voltage forced trip current (ie. 5 amperes). The input resistance of the current sensing circuitry should be low at high currents so that an inordinately high voltage is not required to force the desired trip current and to minimize power losses. Likewise, the input inductance of the current sensing circuitry should be minimal to avoid an adverse effect on trip current rise time. Otherwise the advantage of using a high trip current tends to be negated.
A known method of monitoring the continuity of circuit breaker trip solenoids involves the use of saturable reactors having a D.C. input winding and an A.C. output winding. The A.C. impedance of the output winding increases as current through the input winding decreases. The design should be such that when the input current through the input winding decreases below the desired level of continuity current, the increased impedance of the output winding will cause the triggering of a switch or some other appropriate response which, in turn, for example, may be used to sound an alarm. The only part of the monitoring circuit which is electrically connected to the trip solenoid circuit is the input winding of the saturable reactor connected in series with the trip solenoid.
There are several disadvantages to the use of saturable reactors. Included among these is the fact that the input winding of a saturable reactor necessarily introduces an unwanted inductance in the path of trip current. Also, difficult design problems are encountered. The input winding of the saturable reactor must have the capacity to carry large trip current with relative ease and there must be sufficient turns of this winding to ensure saturation of the reactor in the presence of relatively low continuity current. As the number of turns of input winding is increased, both the input impedance and the physical bulk of the reactor is increased. The use of such a reactor will likely be substantially limited to the particular purpose for which it is designed.
In the foregoing example, the input winding of a saturable reactor formed the basic current sensing element. In many applications, linear resistance means (ie. a current shunt) are commonly used as the current sensing element. But, in certain applications where input current may vary over a wide range, the use of a linear device becomes impractical or virtually impossible. The resistance which the linear resistance must have will be dictated by the minimum current that must be sensed. This resistance may produce voltages beyond a safe or useful level in the presence of high input currents - assuming that such high input currents can be made to flow. Accordingly, the use of a linear resistance may impose restrictions on the range of input currents.
Accordingly, it is an object of the present invention to provide a current sensing circuit arrangement for sensing D.C. current that may vary over a wide range of magnitudes and which permits electrical isolation between the circuit in which sensed current is flowing and an electrically isolated circuit controlled in response to the magnitude of such sensed circuit.
A further object of the present invention is that such circuit arrangement present a relatively high resistance to relatively low D.C. input currents and a relatively low resistance to relatively high D.C. input currents.
Another object of the invention is that such circuit arrangement have a relatively low inductance so as not to unduly impede otherwise rapid changes in the current being sensed.